The present invention relates to an electroluminescent composition and an electroluminescent device having a light-emitting layer of the composition. More particularly, the invention relates to an electroluminescent composition comprising a specific dielectric material as the matrix and a particulate electroluminescent material dispersed in the matrix as well as a dispersion-type electroluminescent device of uniform light emission of high brightness and excellent durability characteristic having a light-emitting layer of the composition.
Different from other types of light-emitting devices, electroluminescent devices in general have a characteristic as a plane light source of low power consumption and are increasingly highlighted in recent years in the applications for illumination and panel display units. Electroluminescent devices are classified depending on the structure of the light-emitting layer into dispersion-type and thin film-type ones, of which the former type ones are now at the starting stage for practical applications as a back of liquid crystal display units, guide light, allnight light and the like by virtue of the advantages such as easiness of preparation, inexpensiveness, shapability into any desired forms and so on.
The light-emitting body of a dispersion-type electroluminescent device is a thin layer formed of a composition comprising a dielectric material as the matrix and a particulate electroluminescent material, such as zinc sulfide, zinc selenide, zinc silicate, boron nitride, silicon carbide and the like, uniformly dispersed in the matrix. The electroluminescent device is prepared basically by coating a substrate to serve as one of the electrodes with a coating liquid which is a dispersion of the particulate electroluminescent material in a solution of the dielectric material to form a thin coating layer and, after evaporation of the solvent, sandwiching the layer with another electrode plate by adhesively bonding with heating under pressure. It is known that the brightness of the electroluminescent layer depends on the dielectric constant of the dielectric material as the matrix and a dielectric material having a larger dielectric constant gives a higher brightness.
The dielectric material of high dielectric constant used for the purpose is usually a polymeric material having a large dielectric constant including cyanoethylated polysaccharides such as cyanoethyl cellulose, cyanoethyl starch, cyanoethyl pullulan and the like, cyanoethylated polysaccharide derivatives such as cyanoethyl hydroxyethyl cellulose, cyanoethyl glycerol pullulan and the like, cyanoethylated polyol compounds such as cyanoethyl poly(vinyl alcohol) and the like and fluorocarbon resins such as poly(vinylidene fluoride) and the like. These conventional high-dielectric polymeric materials, however, have several disadvantages and problems and are not always quite satisfactory as a matrix material for particulate electroluminescent materials.
For example, the cyanoethylated polysaccharides and polysaccharide derivatives and cyanoethyl poly(vinyl alcohol) have a defect in common in the large hygroscopicity. When such a polymeric material is used as the matrix of particulate electroluminescent materials, namely, the electroluminescent material is subject to degradation of the light-emitting efficiency as being affected by the large moisture content of the matrix so that the durability of the electroluminescent device is unavoidably decreased. This undesirable phenomenon can be prevented to some extent by taking a measure for rigorous moisture control and dehumidification of the matrix material in the manufacturing process of the devices although such a measure is undesirable in respect of the productivity of the manufacturing process if not to mention the incompleteness of the effect obtained thereby.
Besides, cyanoethyl cellulose and cyanoethyl starch have a problem in the relatively low adhesive bonding strength to the substrate electrodes sometimes to form a void interstice between the electrode and the light-emitting layer or to cause portion-wise exfoliation of the electrode and the light-emitting layer so that the uniformity in the light emission as one of the characteristic features of electroluminescent devices is greatly decreased. This problem of low adhesive bonding strength can be partly solved by admixing the matrix polymer with a plasticizer but admixture of a plasticizer necessarily results in a decreased dielectric constant and consequent decrease in the brightness as well as a decrease in the durability of the light-emitting body.
Another serious problem in the cyanoethyl hydroxyethyl cellulose, cyanoethyl glycerol pullulan and cyanoethyl poly(vinyl alcohol) is the relatively large temperature dependency of the dielectric constant thereof which sometimes limits the temperature range in which the electroluminescent devices using these polymers as the matrix can be used.
Fluorocarbon resins such as poly(vinylidene fluoride) and the like, on the other hand, are advantageous in respect of their low hygroscopicity and small temperature dependency of their dielectric constant. However, the dielectric constant of the polymers of this class in general is as small as only about a half of that of the cyanoethylated polysaccharides and polysaccharide derivatives so that the brightness of the devices using such a polymer inherently cannot be high enough.